RFR: 8359419: AArch64: Relax min vector length to 32-bit for short vectors [v3]

[Xiaohong Gong]

On Sat, 5 Jul 2025 15:08:35 GMT, Fei Gao <fgao at openjdk.org> wrote:

> Have you measured the performance of this micro-benchmark on NEON machine?
> 
> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/test/micro/org/openjdk/bench/vm/compiler/TypeVectorOperations.java#L251-L256
> 
> We added an limitation only for `int` before:
> 
> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/cpu/aarch64/aarch64_vector.ad#L131-L134
> 
> Perhaps we also need to impose a similar limitation on `short` if the same regression occurs.

Good catch, and thanks so much for your input @fg1417 ! I will test the performance and disable auto-vectorization for double to short casting if the performance has regression. 

> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/test/hotspot/jtreg/compiler/vectorization/runner/ArrayTypeConvertTest.java#L388-L392

Actually I didn't change the min vector size for `char` vectors in this patch. Relaxing `short` vectors to 32-bit is to support the vector cast for Vector API, and there is no `char` species in it. Do you think it's better to do the same change for `char` as well? This will just benefit auto-vectorization.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/26057#issuecomment-3043711086